Not so long ago, WWDC was all about Mac OS X, the desktop operating system that's now more than a dozen years old and in it's ninth iteration (OS X 10.8).

In 2001, Steve Jobs introduced the BSD-based Mac OS X as the future of the company and said Apple would use it as the foundation for its computers for the next 15 years. Six years later, Mac OS X was stripped of its non-essential elements and turned into an OS for mobile devices - the iPhone and iPod touch in 2007, and the iPad in 2010.

With the arrival of iOS, Apple gave us something we didn't yet know we wanted - or needed: a way to carry a computer in a pocket. That was the genius of the original iPhone; Apple's new device was less of a phone with computer-like functions, and more like a computer that also worked as a phone. For most people, using the new iPhone didn't feel like computing at all.

OS X begets iOS

Apple took what it learned in developing OS X for the desktop - how things looked, how they worked, moved and responded - and used that UI expertise to connect iPhone owners to software in a way that had never been done before. Suddenly, swiping and tapping was all the rage.

iOS reached people in a way OS X never could, with touch, and it was that visceral connection that helped launch the iPhone - and later the iPad - into the stratosphere. Apple was no longer just a computer company; it was a mobile computer company leading the way toward the post-PC world.

Almost as importantly, the iPhone and iPad helped break through the price barrier that had for so long kept computer users from buying Apple products. Although the first iPhone was priced at $600 initially, newer versions like the still-available iPhone 4 start at $0 (with a two-year contract), and the cheapest iPhone 5 is only $199.

The more popular the platform, the bigger the audience -- and the more likely it is that the platform will grow. Developers know this, and act accordingly. That's why the Apple App Store now has nearly 900,000 apps - many of which are quite good. Apple knows this, too, which is why iOS 7, slated to be previewed at WWDC on Monday -- carries on its shoulders a heavy responsibility. It has to serve as both a rock-solid foundation for developers and a robust OS for users.

iOS, like all mobile OSes, needs to be responsive and stable -- it generally is already - but it does need better third-party iCloud syncing support. It also sounds as though, based on recent speculation about UI changes, we'll see lead designer Jony Ive's new take on the overall interface. That should make iOS look more like 2013 and less like 2007.

The Apple ecosystem

How important is iOS in the Apple ecosystem? Look at it this way: Apple has more than 500 million active iOS user accounts, with some analysts predicting there will be more than 600 million by year's end. That enormous iOS base - most of whom are using iOS 6 already - offer a compelling siren song to developers looking to make a living. And it creates a secondary market of connected devices that continues to grow by leaps and bounds.

As a result, the variety of Stuff That Can Be Done using iOS and the apps built for it is telling.

There are countless examples of useful and cool things you can now do with iOS that were never an option for OS X. For instance, I'm into fitness (sometimes) and the breadth of hardware add-ons and software available for iOS, just in the Health- and Activity-tracking categories alone, is enough to make my head spin.

Even better? The hardware comes with dedicated iOS apps that are capable of syncing results with their respective online services. Those online services take things even further by tying themselves into other, sometimes competing, online services, allowing for cross-application data-sharing.

The TactioHealth app, as an example, takes all of my data and stores it in one place, making it easy to track my health over time, offering graphs with healthy/unhealthy zones, and providing simple instructions on how to improve my scores.

Certainly, I didn't need an iPad attachment to tell me I had high blood pressure, but the actual numbers (without a doctor's visit) are good to know, and the information has actually motivated me to do better.

This is the power of a platform that has reached critical mass: there are more possibilities than you know what to do with. At this point, all categories are covered. Want to remote start your car? There's an app and service for that. Home automation? Same. Reading? Gaming? Entertainment? It's all here, and available now.

We're really only at the beginning of how mobile connectedness is going to change how we live, work and relax. Just as the Mac was positioned as the Digital Hub for other electronics, iOS allows our iPhones and iPads and iPods to interact with other devices that wouldn't be feasible with traditional desktops or laptops. In all my years of using Apple computers, I've never seen anything like the explosion of accessories, hardware, and software that's been seen in the last six years.

But, then again, the Mac never had 500 million unique users.

OS X still matters

So, it makes sense for Apple's leadership to shift resources - developers and money -- away from desktops and laptops and the operating system they use. That, of course, doesn't mean that the Mac will be left behind. The Apple ecosystem relies heavily on the integration of Apple hardware, software and services, so it behooves Apple to keep pushing its mobile and traditional platforms. That's why rumors are spreading that updated MacBook Air laptops sporting Intel's new Haswell chips may arrive next week, along with preview versions of OS X 10.9 and iOS 7.

Basically, the success of iOS feeds back to OS X - which, of course, took on some of the mobile operating system's features two years ago - and will ultimately boost Apple's hardware line-up. It's a familiar phenomenon that even has a name: the Halo Effect. In essence, each Apple device serves to entice users to try out other Apple products.

What's interesting is the expanding ecosystem of hardware, apps and services that iOS will enable in the future. If my experience with health devices and apps is an indicator, the possibilities are almost endless: with computers in our pockets, on our clothes, on our wrists, and built into eyewear, it's now possible to imagine connections and integrations that go far beyond a notebook or desktop. (I'm now convinced that Apple is working on a sensor-laden wrist accessory and the software to support it, despite my initial skepticism. In fact, Apple CEO Tim Cook hinted at that prospect just last week.)

Even though we no longer live in a world of standalone devices, there are few companies selling device interaction the way Apple is now doing. You can see the plan unfold: The iPhone and iPad lead the way through a field of ever-converging devices and data, using iCloud services to keep you and your hardware and your information ever connected. All of it. Anytime. Anywhere. Whether it's an X-ray for your doctor, a movie you want to finish watching. A document that you need to send by email. Or your latest workout.

Jobs turned out to be right when he bet the future of Apple on OS X 12 years ago, because iOS is nothing more than OS X evolved, a modern take on an operating system that continues to deliver on the desktop, even as the times pass it by.

And that's why iOS is so important: because it powers the computer you always have with you. The Mac may be the Digital Hub for your traditional peripherals, but the iPhone is truly at the center of today's modern lifestyle. With its low cost, array of sensors and wireless connection options, these pocket computers will be used to connect hardware, services, and people in a way that never made sense with traditional computers.

Simply put, iOS is the future. And that's why when Cook and other Apple executives take the stage on Monday, they'll surely talk about OS X 10.9 and whatever improvements they have in mind for it. But the future they envision depends on iOS 7 and its successors.

Michael deAgonia, a contributing writer for Computerworld, is a computer consultant and technology geek who has been working on computers since 1993, with an emphasis on Macs, macOS, and iOS. For more Apple insights, follow him on Twitter.